Conventional devices used for measuring sheet resistivity are limited in their measurement range and accuracy by variances in probe pressure, probe spacing and by the circuits employed as a current source and a voltage amplifier. Means for ensuring uniform pin pressure on a sample is required since data consistency is directly affected by the load or force with which each probe is pressed into a sample. The current source must have very high output resistance in order to maintain a selected current value over a wide range of voltages and a large voltage range is required not only by the varied resistances seen between the outer two probes, but also by the potential necessary to achieve avalanche breakdown of point-contact diodes formed when the probes touch lightly-doped wafers. These diodes have typically made the accurate measurement of high resistivity wafers difficult, if not impossible. For low-resistivity wafers, the voltage signal must usually be highly amplified; for example, gains of 10.sup.5 are not uncommon. Drift in a d.c. amplifier's input offset voltage due to time and temperature variations can mask the actual data when such low magnitude voltages are of interest. Thus, the amplifier must be very stable, or some means of correcting the drift errors must be provided. For high-resistivity wafers, point-contact diodes are also formed at the probes. Since all practical amplifiers require some input bias current, the voltage drop required across these diodes in order to allow this bias current flow produces errors at the input of the voltage amplifier. Some means must be provided for correcting these errors.
The value of sheet resistivity is typically found by dividing voltage by a current and multiplying the result by a constant. Conventional systems set the current equal to this constant. Such a technique does not necessarily yield the most desirable value of current. Furthermore, if various wafer geometries are to be accepted, the constant and thus the current must be variable. This leads to difficulty in accurately setting the current. A better method would be to set the current at the desired value and make the constant adjustable by actually performing a data multiplication.